Aspartyl residues in enamel and dentine isolated from human teeth undergo slow racemization with age, while no such age-related increase has been detected in hemoglobin. These observations suggest that racemization of aspartyl residues occurs in metabolically stable proteins over time and that this phenomenon might be used to estimate the in vivo lifetimes of structural proteins and ages of living mammals. We now propose to investigate the occurrence of racemization in proteins of the human lens nucleus and in collagen isolated from various connective tissues. If racemization is detected in connective tissue, then turnover rates can be estimated and pepsin-solubilized collagen will be analyzed in order to localize where racemization has occurred: in the helical portion or in the random coil regions of the molecule. If racemization is detected in the helical portion of the collagen molecule, spectral measurements may elucidate what effects increasing amounts of D-aspartyl residues have on helical conformation. Should racemization not be detected in any soft tissue protein, we will then fractionate human dentine in order to determine whether the aspartyl residues in a particular non-collagenous protein component (phosphoprotein) are more susceptible to racemization than are those in collagen or whether some feature of the mineral phase facilitates racemization. Analyses will be carried out on the phosphoprotein fraction and on cyanogen bromide peptides of the collagen fraction. One intriguing application of the racemization of aspartic acid in human dentine will be to try to test the claims of unusual longevity of various human populations, provided teeth suitable for analysis can be obtained.